A gas turbine installed as an aircraft engine comprises a compressor compressing ambient air, a combustor burning fuel together with the compressed air and a turbine for powering the compressor. The expanding combustion gases drive the turbine and also result in thrust used for propelling the air craft.
Gas turbines engines consume large quantities of air. Air contains foreign particles in form of aerosols which enters the gas turbine compressor with the air stream. The majority of the foreign particles will follow the gas path and exit the engine with the exhaust gases. However, there are particles with properties of sticking on to components in the compressor's gas path. Stationary gas turbines like gas turbines used in power generation can be equipped with filter for filtering the air to the compressor. However, gas turbines installed in aircrafts are not equipped with filters because it would create a substantial fall in pressure and are thereby more exposed to air contaminants. Typical contaminants found in the aerodrome environment are pollen, insects, engine exhaust, leaking engine oil, hydrocarbons coming from industrial activities, salt coming from nearby sea, chemicals coming from aircraft de-icing and airport ground material such as dust.
Preferably engine components such as compressor blades and vanes should be polished and shiny. However, after a period of operation a coating of foreign particles builds up. This is also known as compressor fouling. Compressor fouling results in a change in the properties of the boundary layer air stream of the components. The deposits result in an increase of the component surface roughness. As air flows over the component surface the increase of surface roughness results in a thickening of the boundary layer air stream. The thickening of the boundary layer air stream has negative effects on the compressor aerodynamics. At the blade trailing edge the air stream forms a wake. The wake is a vortex type of turbulence with a negative impact on the air flow. The thicker the boundary layer the stronger the turbulence in the wake. The wake turbulence together with the thicker boundary layer has the consequence of a reduced mass flow through the engine. The reduced mass flow is the most profound effect of compressor fouling. Further, the thicker boundary layer and the stronger wake turbulence formed at the blade trailing edge result in a reduced compression pressure gain which in turn results in the engine operating at a reduced pressure ratio. Anyone skilled in the art of heat engine working cycles understands that a reduced pressure ratio result in a lower thermodynamic efficiency of the engine. The reduction in pressure gain is the second most remarkable effect from compressor fouling. The compressor fouling not only reduces the mass flow and pressure gain but also reduces the compressor isentropic efficiency. Reduced compressor efficiency means that the compressor requires more power for compressing the same amount of air. The reduced mass flow, pressure ratio and isentropic efficiency reduce the engine thrust capability. The power for driving the compressor is taken from the turbine via the shaft. With the turbine requiring more power to drive the compressor there will be less thrust for propulsion. For the air craft pilot this means he must throttle for more power as to compensate for the lost thrust. Throttling for more power means the consumption of fuel increases and thereby increasing operating costs.
Compressor fouling also has a negative effect to the environment. With the increase of fuel consumption follows an increase of emissions of green house gas such as carbon dioxide. Typically combustion of 1 kg of aviation fuel results in formation of 3.1 kg carbon dioxide.
The loss in performance caused by compressor fouling also reduces the durability of the engine. As more fuel has to be fired for acquiring a required thrust, follows an increase in the temperature in the engine combustor chamber. When the pilot throttles for take-off on the runway the temperature in the combustion chamber is very high. The temperature is not too far from the limit of what the material can stand. Controlling this temperature is a key issue in engine performance monitoring. The temperature is measured with a sensor in the hot gas path section downstream of the combustor outlet. This is known as exhaust gas temperature (EGT) and is carefully monitored. Both exposure time and temperature are logged. During the lifetime of the engine the EGT log is frequently reviewed. At a certain point of the EGT record it is required that the engine will have to be taken out of service for an overhaul.
High combustor temperature has a negative effect to the environment. With the increase of combustor temperature follows an increase of NOx formation. NOx formation depends to a large extent on the design of the burner. However, any incremental temperature to a given burner results in an incremental increase in NOx.
Hence, compressor fouling has significant negative effects to aero engine performance such as increased fuel consumption, reduced engine life, increased emissions of carbon dioxide and NOx.
Jet engines can have a number of different designs but the above-mentioned problems arises in all of them. Typical small engines are the turbojet, turboshaft and turboprop engines. Other variants of these engines are the two compressor turbojet and the boosted turboshaft engine. Among the larger engines there are the mixed flow turbofan and the unmixed flow turbofan which both can be designed as one, two or three shaft machines. The working principles of these engines will not be described here.
The turbofan engine is designed for providing a high thrust for aircraft operating at subsonic velocities. It has therefore found a wide use as engines for commercial passenger aircrafts. The turbofan engine comprises of a fan and a core engine. The fan is driven by the power from the core engine. The core engine is a gas turbine engine designed such that power for driving the fan is taken from a core engine shaft. The fan is installed upstream of the engine compressor. The fan consists of one rotor disc with rotor blades and alternatively a set of stator vanes downstream if the rotor. Prime air enters the fan. A discussed above, the fan is subject to fouling by insects, pollen as well as residue from bird impact, etc. The fan fouling may be removed by washing using cold or hot water only. This cleaning washing process is relatively easy to perform.
Downstream of the fan is the core engine compressor. Significant for the compressor is that it compresses the air to high pressure ratios. With the compression work follows a temperature rise. The temperature rise in a high pressure compressor may be as high as 500 degree Celsius. We find that the compressor is subject to different kind of fouling compared to the fan. The high temperature results in particles more easily being “baked” to the surface and will be more difficult to remove. Analyses show that fouling found in core engine compressors are typically hydrocarbons, residues from anti-icing fluids, salt etc. This fouling is more difficult to remove. It may at some time be accomplished by washing with cold or hot water only. Else the use of chemicals will have to be practised.
A number of cleaning or washing techniques have been developed during the years. In principle, aero engine washing can be practised by taking a garden hose and spraying water into the engine inlet. This method has however a limited success due to the simple nature of the process. An alternative method is by hand scrubbing the compressor blades and vanes with a brush and liquid. This method has limited success as it does not enable cleaning of the interior blades of the compressor. Moreover, it is time-consuming. U.S. Pat. No. 6,394,108 to Butler discloses a thin flexible hose which one end is inserted from the compressor inlet towards the compressor outlet in between the compressor blades. At the inserted end of the hose there is a nozzle. The hose is slowly retracted out of the compressor while liquid is being pumped into the hose and sprayed through the nozzle. The patent discloses how washing is accomplished. However the washing efficiency is limited by the compressor rotor not being able to rotate during washing. U.S. Pat. No. 4,059,123 to Bartos discloses a mobile cart for turbine washing. However, the patent does not disclose how the cleaning process is accomplished. U.S. Pat. No. 4,834,912 to Hodgens II et al. discloses a cleaning composition for chemically dislodging deposits of a gas turbine engine. The patent illustrates the injection of the liquid into a fighter jet aircraft engine. However, no information is provided about the washing process. U.S. Pat. No. 5,868,860 to Asplund discloses the use of a manifold for aero engines with inlet guide vanes and another manifold for engines without inlet guide vanes. Further the patent discloses the use of high liquid pressure as means of providing a high liquid velocity, which will enhance the cleaning efficiency. However, the patent does not address the specific issues related to fouling and washing of turbofan aero engines.
The arrangement described hereinafter with reference to FIG. 1 is further regarded as common knowledge in this field. A cross section view of a single shaft turbojet engine is shown in FIG. 1. Arrows show the mass flow through the engine. Engine 1 is built around a rotor shaft 17 which at its front end is connected a compressor 12 and at its rear end a turbine 14. In front of the compressor 12 is a cone 104 arranged to split the airflow. The cone 104 is not rotating. The compressor has an inlet 18 and an outlet 19. Fuel is burnt in a combustor 13 where the hot exhaust gases drives turbine 14.
A washing device consist of a manifold 102 in form of a tube which in one end is connected to a nozzle 15 and the other end connected to a coupling 103. Hose 101 is at one end connected to coupling 103 while the other end is connected to a pump (not shown). Manifold 102 is resting upon cone 104 and is thereby held in a firm position during the cleaning procedure. The pump pumps a washing liquid to nozzle 15 where it atomizes and forms a spray 16. The orifice geometry of nozzle 15 defines the spray shape. The spray can form many shapes such as circular, elliptical or rectangular depending on its design. For example, a circular spray has a circular distribution of droplets characterized by the spray having the shaped of a cone. An elliptical spray is characterised by one of the ellipses axis is longer than the other. A rectangular spray is somewhat similar to the elliptical spray but with corners according to the definition of a rectangle. A square spray is somewhat similar to the circular spray in that the two geometry axes are of equal length but the square shaped spray has corners according to the definition of a square.
Liquid is atomized prior to entering the compressor for enhanced penetration into the compressor. Once inside the compressor the droplets collide with gas path components such as rotor blades and stator vanes. The impingement of the droplets results in wetting surface and establishing of a liquid film. The deposited particles on the gas path components are released by mechanical and chemical act of the liquid. Liquid penetration into the compressor is further enhanced by allowing the rotor shaft to rotate during washing. This is done by letting the engine's starter motor turn the rotor whereby air is driven through the engine carrying the liquid from the compressor inlet towards the outlet. The cleaning effect is further enhanced by the rotation of the rotor as the wetting of the blades creates a liquid film which will be subject to motion forces such as centrifugal forces during washing.
What is said about the cleaning of the compressor will also have effect on cleaning of the whole gas turbine engine. As the cleaning liquid enters the engine compressor and the rotor is rotating the washing fluid will enter the combustion chamber and further through the turbine section and thereby cleaning the whole engine.
However, this method is not efficient for a turbofan turbine engine for a number of reasons. Firstly, because the fouling of different components of a turbofan engines may have significantly different properties regarding, for example, the stickiness, it will require different methods for the removal as discussed above. Secondly, since the fan and its cone for splitting the airflow is rotating, the cone cannot be used for holding the manifold. Possible, the manifold can be mounted on a stand or a frame placed upstream of the fan but this arrangement would not provide an efficient cleaning of the engine since the main part of the cleaning liquid emanated from the nozzles would impinge at the suction side of the blades of the fan.